The present invention relates to an air fuel ratio controlling device of an internal combustion engine. More particularly, the present invention relates to an identification facility for rich and lean at starting of the closed loop control in this system.
As a means for decreasing harmful exhaust gas of an automobile, it has been proposed recently an air fuel ratio controlling device based on a feedback control principle, in which the air fuel ratio is controlled based on an information concerning the concentration of the exhaust gas content of the engine.
FIG. 1 shows one embodiment of such a feedback system. In this system, the concentration of the exhaust gas content, including for instance O.sub.2, CO, CO.sub.2, HC, NO.sub.x, etc. is detected by an exhaust gas sensor 3 provided in the exhaust pipe 2. The output signal of the sensor 3 is compared with a reference value V.sub.s, which is for instance a value corresponding to a settled air fuel ratio in a deviation detecting circuit 4 consisting for instance of a differential amplifier, a comparator, etc. and the deviation is detected. By using a control circuit 5, a control signal corresponding to said deviation is produced. This control signal may be an electric signal being in proportion to the deviation, an integrated signal being an integration of the deviations, or a signal being an addition of the both signals. The amount of the fuel supply and the air supply may be additionally controlled by using the above control signal by a fuel amount adjusting or controlling device 6, which may be a carburetter, or a fuel injection means, so that the air fuel ratio of the mixed gas to be supplied to the engine 1 is maintained at a predetermined settled ratio. It is obvious that the fuel amount adjusting device 6 is controlled by a separate factor such as control of the throttle valve by the driver.
If the settled value of the air fuel ratio is adjusted to be one to match with the most suitable operational point of the exhaust gas purifier 7, which may be a catalyst means, a reactor or the like, the harmful component in the exhaust gas can efficiently be decreased.
If a ternary catalyst device, which acts to oxidize CO and HC and simultaneously acts to reduce NO.sub.x, the settled air fuel ratio is set at a value near a stoichiometric air fuel ratio.
The exhaust gas sensor used in the aforementioned air fuel ratio control device generally varies its characteristics by the temperature. For instance in a Zirconia oxygen concentration meter widely used in this field has an equivalent electric circuit as shown in FIG. 2. This equivalent circuit comprises a series arrangement of a battery having varying electromotive force e according to the oxygen concentration and an internal resistance having varying internal resistance .rho. according to the temperature. The value of the internal resistance .rho. has a temperature characteristic as shown in FIG. 3. As can be seen from FIG. 3, the internal resistance .rho. becomes a large value at low temperature so that it becomes difficult to derive the electromotive force e. Due to this feature, the air fuel ratio controlling device is required to operate under open loop control, in general to keep it at a certain condition, at a time of low temperature of the exhaust gas sensor and to shift it to the closed loop control, i.e. feedback control, after the exhaust gas sensor becomes a temperature sufficiently high for its operation.
As a method to detect the temperature of the exhaust gas sensor it has been known to use variation of the internal resistance. In this method, a current is applied to flow through the exhaust gas sensor and the voltage variation according to the variation of the internal resistance by the temperature change is detected.
If a certain current i is assumed to flow in the exhaust gas sensor, the output voltage V.sub.O of the exhaust gas sensor can be expressed as follows. EQU V.sub.O =e +.rho.i (1)
In the above equation (1), if the internal resistance .rho. becomes small according to the temperature rise, the voltage V.sub.O also decreases. Therefore, the closed loop control may be started when V.sub.O becomes less than a certain value.
However, there is still difficult problem at the time of start of the closed loop control.
It is known that the reference value V.sub.s in the deviation detecting circuit 4 is better to be varied according to the condition of the output of the exhaust gas sensor rather than to fix it at a certain fixed value. In this case, a benefit is obtained in that the variation of the output of the exhaust gas sensor may effectively be compensated at the time of low temperature or at deterioration of the sensor.
As the method for varying the reference value V.sub.s according to the condition of the output of the exhaust gas sensor, it has been known to take a mean value of the largest value (value under too rich mixed gas) and the smallest value (value under too lean mixed gas) and the mean value is used as the reference value V.sub.s. However, before starting the closed loop control as mentioned in the foregoing, the output of the exhaust gas sensor is either the largest value or the smallest value so that it is impossible to obtain the mean value between the largest and the smallest values at such time before starting the closed loop control. Accordingly, there was considered a system to decide the ference value V.sub.s at a value decreased by a certain amount from the largest value if the output of the exhaust gas sensor is the largest one at the time before the start of the closed loop control and at a value increased by a certain amount from the smallest value if the output is the smallest. Therefore the mean value of the largest and smallest values of the sensor output is used as the reference value. According to this system, the reference value V.sub.s always lies in a range between the largest and the smallest values so that a proper feedback control is effectied automatically after starting thereof.
However, the above system still has a problem in that an accurate judgement is requested whether the output of the exhaust gas sensor is the largest or the smallest value, or in other words whether the mixed gas is rich or lean before starting of the closed loop control. If the reference value is erroneously increased even the mixed gas is rich, the mixed gas is deviated towards rich side and in the opposite case it is deviated towards lean side so that a proper feedback control cannot be realized.
For solving the aforementioned problem, a further method had been considered which will be explained by referring to FIG. 4.
FIG. 4 is a diagram for showing a relation between the output voltage of the exhaust gas sensor, the temperature and the internal resistance thereof in a system in which an outer supply current is fed therethrough.
In FIG. 4, a curve X shows an output voltage of the sensor applied with the current and when the mixed gas is too rich, a curve X' shows an output voltage in which the outside current supply is discontinued at the above time, a curve Y shows an output voltage of the sensor applied with the current and when the mixed gas is too lean, and a curve Y' shows an output voltage when the current is discontinued at the above time.
We assume that the starting time for the closed loop control is at a time when the output voltage V.sub.O being applied with current becomes as. EQU V.sub.O .ltoreq.V.sub.p1 (For instance V.sub.p1 may be 1.2 V)
Namely the above condition exists at a time when either the curve X or the curve Y becomes below the settled value V.sub.p1.
In this case if we discontinue the current supply at a time when the curve X or the curve Y crosses the settled value V.sub.p1, the output voltage V.sub.O decreases from V.sub.x to V.sub.y. Since there is a considerable difference between the values V.sub.x and V.sub.y so that a second settled value V.sub.p2 is determined between the curves X' and Y' and the output voltage V.sub.O when cutting off the current supply is compared with the value V.sub.p2. By this way the rich or lean of the mixed gas then is judged more accurately. Based on the result of the judgement, the reference value is further adjusted. Namely, if the mixed gas is too rich, a value lower than V.sub.p1 by a certain value is taken up to be the reference value V.sub.s and if it is too lean a value higher than V.sub.p1 by a certain value is taken up as the reference value V.sub.s. Then the current is again applied. In this case, the reference value V.sub.s always becomes between the curve X and the curve Y. The automatic feedback control is effected properly thereafter.
However, this system must have a quite complicated procedure in that the current is discontinued after a judgement for the possibility of start of closed loop control, that a judgement for too rich or too lean is made, that a reference value V.sub.s is decided based on the same and that the closed loop control is started after the current is supplied again. Moreover when the current is switched ON or OFF condition, it requires a certain time before the output voltage V.sub.O varies from the curves X, Y to curves X', Y' or reversely (a time during which the engine may rotate some tens of times) so that the judgement should be delayed over the above period. This means that the starting time of the closed loop control is delayed for the above period and the exhaust gas purifying characteristics deteriorate during the above period.